The Periodic Table of Elements as adopted by the American Chemical Society is employed in designating elements.
The term "superconductivity" is applied to the phenomenon of immeasurably low electrical resistance exhibited by materials. Until recently, superconductivity had been reproducibly demonstrated only at temperatures near absolute zero. As a material capable of exhibiting superconductivity is cooled, a temperature is reached at which resistivity decreases (conductivity increases) markedly as a function of further decrease in temperature. This is referred to as the superconducting transition temperature or, in the context of superconductivity investigations, simply as the critical temperature (T.sub.c). T.sub.c provides a conveniently identified and generally accepted reference point for marking the onset of superconductivity and providing temperature rankings of superconductivity for different materials. The highest temperature at which superconductivity is observed in a material is designated T.sub.o. Materials which exhibit superconductivity at temperatures in excess of 30.degree. K. are herein referred to as high temperature superconductors.
A variety of compounds containing copper and other metals in combination with oxygen have been observed to be electrically conductive in crystalline form and to be high temperature superconductors. These materials are herein referred to collectively as "conductive crystalline cuprates". Electrically conductive articles including conductive crystalline cuprate layers exhibiting higher temperature superconductivity are illustrated by the following:
R-1 Mir et al published European Patent Application No. 290,357A (priority based on U.S. Ser. No. 46,593, filed May 4, 1987 now U.S. Pat. No. 4,880,770) discloses thin (&lt;5 .mu.m) conductive crystalline cuprate layers formed on refractory substrates at temperatures of at least 900.degree. C. Substrates disclosed for use are alkaline earth oxides (e.g., alumina, magnesia, and strontium titanate) and semiconductors (e.g., silicon and 3-5 compounds). The substrates can include barrier layers interposed between the foregoing substrate portions and the conductive crystalline cuprate layer, the barrier layers disclosed include silica, silicon nitride, and refractory metals (e.g., tantalum, titanium, and zirconium).
R-2 Strom et al published European Patent Application 297,319 (priority based on U.S. Ser. No. 68,391, filed July 1, 1987, now U.S. Pat. No. 4,908,346) contains a disclosure similar to that of R-1, except that a different process of coating is disclosed to produce thick (&gt;5 .mu.m) conductive crystalline cuprate layers.
R-3 Agostinelli et al published European Patent Application No. 303,083 (priority based on U.S. Ser. No. 85,047, filed Aug. 13, 1987 now abandoned, and U.S. Ser. No. 46,593, cited above) contains a disclosure similar to that of R-1 and R-2, but requires a barrier layer containing a metal in its elemental form or in the form of an oxide or silicide chosen from the group consisting of magnesium, a group 4 metal, or a platinum group metal.
R-4 Hung et al U.S. Ser. No. 153,699, filed Feb. 8, 1988, now U.S. Pat. No. 4,908,348, contains a disclosure similar to that R-1 and R-2, except that a silicon substrate portion is separated from the cuprate layer by a barrier layer triad. The barrier layer triad is comprised of a first triad layer located adjacent the silicon substrate portion consisting essentially of silica, a third triad layer remote from the silicon substrate portion consisting essentially of one Group 4 heavy metal oxide, and a second triad layer interposed between the first and third triad layers consisting essentially of a mixture of silica and at least one Group 4 heavy metal oxide.
R-5 Agostinelli et al U.S. Ser. No. 214,976, filed July 5, 1988 (as a continuation in part of U.S. Ser. No. 172,926, filed March 25, 1988, now abandoned in favor of U.S. Ser. No. 359,306, filed May 31, 1989) contains a disclosure similar to R-1, R-2, R-3, and R-4, but employs as a conductive crystalline cuprate layer a crystalline heavy pnictide mixed alkaline earth copper oxide wherein the heavy pnictide is bismuth and optionally contains antimony in a concentration of less than 10 mole percent based upon bismuth. In a preferred form the cuprate is similar to that disclosed by H. Maeda, Y. Tanaka, M. Fukutom, and Y. Asano, "A New High T.sub.c Superconductor Without a Rare Earth Element", Japanese Journal of Applied Physics, Vol. 27, No. 2, pp. L209 &L210. Crystallization at temperatures as low as 800.degree. C. are disclosed.
R-6 Agostinelli et al U.S. Ser. No. 208,706, filed June 20, 1988, contains a disclosure similar to the preceding, but requires the presence of a metal or metal oxide (copper or copper oxide) layer interposed between the substrate and the cuprate layer. The interposed layer facilitates clean etchant removal of the cuprate from the substrate.
R-7 Lelental et al U.S. Ser. No. 208,707, filed June 20, 1988, adds to the preceding disclosures a technique for forming a mixture of (1:2:3) and (2:1:1) rare earth alkaline earth cuprate phases.
R-8 Lelental et al U.S. Ser. No. 236,420, filed Aug. 25, 1988, discloses an improvement on the technique of R-7.
R-9 Chatterjee U.S. Ser. No. 290,670 filed, Dec. 27, 1988, discloses the protection of the conductive cuprates from degradation by contact with ambient air by employing a polyester ionomer or alkyl cellulose as a passivant.
R-10 Strom U.S. Ser. No. 291,921 filed Dec. 29, 1988, is similar to R-2, but employs a bismuth containing conductive cuprate of the type disclosed by R-5.
All of the copending patent applications are commonly assigned. The foregoing are here incorporated by reference.
From the foregoing it is apparent that electrically conductive articles containing conductive crystalline cuprate layers have in ea:h instance been prepared by firing a cuprate layer to its crystallization temperature on a substrate (optionally including one or more barrier layers) selected from a relatively limited class of inorganic materials. The substrate materials must withstand the high firing temperatures, must not release ions that excessively degrade the conduction properties of the cuprate layer during, and must provide a surface for the cuprate layer that is conducive to crystal formation and growth.
None of the conductive crystalline cuprate layer and substrate combinations heretofore successfully produced have satisfied the need for electrically conductive, particularly high temperature superconductive, articles having a high level of flexibility. For example, none of these articles have exhibited the flexibility of electrically conductive articles employing organic (e.g., polymeric) film supports. The reason for this is quite obvious. The minimum crystallization firing temperatures plausibly conceivable for the fabrication of conductive crystalline cuprate layers are still well in excess of the maximum temperatures at which organic compounds, including even the most thermally stable polymers, possess dimension stability. The overwhelming majority of organic materials decompose at temperatures below 300.degree. C. This is well below the lowest reported crystallization temperature of a conductive cuprate layer and more than 400.degree. C. below typical cuprate crystallization temperatures.